Current and next generation optical networks are capable of transporting multiple wavelengths on the same fiber by using Dense Wavelength Division Multiplexing (DWDM) technology. Typical systems are capable of transporting thirty-two or more wavelength channels, at ten Gigabits per second (Gb/s) rate each. With capacities exceeding 320 Gb/s per fiber, it is becoming increasingly efficient and economical to perform protection and restoration of traffic in the optical layer. In fact, a major network failure, such a fiber cut or node failure, would impact an extremely large number of client layer devices (e.g., ATM switches or IP routers), making service layer protection intractable.
Many networks today are based upon fiber-ring architectures, as evidenced by the proliferation of SONET/SDH time-division multiplexing (TDM) rings all the way from the long-haul backbone to the metropolitan and regional areas. Most large backbone rings represent significant investments on the part of service providers, and expectedly will have longer lifetimes. As a result, ring architectures will clearly play a major role in the evolution of optical networks. Given this large, entrenched base of ring topologies, currently many optical communication network operators are planning for a migration to equivalent dynamic optical ring architectures. Dynamic optical rings can be defined as fiber rings with dynamic light-path provisioning capabilities (such as routing, add/drop and protection). These optical wavelength routing rings, commonly also referred to as optical add-drop ring multiplexer (O-ADM) rings, will form the mainstay architecture for most metro/regional and even long-haul networks, helping operators ease their transition to future optical (mesh or hybrid ring-mesh) networks.
Since many operators have significant experience in deploying and maintaining SONET/SDH rings, future optical analogs of such TDM ring switching are of great transitional value. In optical rings, wavelength channels (as opposed to TDM circuits) undergo bypass, add, or drop operations at ring network elements.
A need exists for fast, scalable optical layer protection/restoration mechanisms. Individual channels (i.e., timeslots) in SONET/SDH rings (e.g., in Bidirectional Line Switching Ring or BLSR architectures) can be restored in 50 ms in the case of a “clean” ring that does not carry extra traffic, or in 100 ms if extra traffic is present in the ring.
Undoubtedly, optical ring solutions must provide equivalent, or improved, capabilities in order to replace SONET/SDH rings in a timely manner. Since each fiber (or wavelength) in an optical network can now carry a much higher degree of multiplexed traffic, Automatic Protection Switching (APS) capabilities are even more crucial
It is also of paramount importance for any optical layer protection mechanism that the mechanism be scalable. In other words, the end-to-end restoration time must depend as little as possible on (and ideally must be independent of) the number of nodes in the ring, and of the number of wavelengths that the ring carries, and that in a worse case scenario might need to be restored.
OCh/SPRING Architectures
Optical Channel Shared Protection Ring (OCh/SPRING) architectures provide a protection mechanism that can protect each optical channel individually based on optical channel failure indications. In OCh/SPRING, protection resources (e.g., wavelengths around the ring) can be shared. That is, the same protection wavelength can be used to protect multiple disjoint working channels. In addition, extra traffic (such as unprotected, pre-emptable traffic) can be provisioned on the protection wavelengths; under normal network fault-free condition, both working and extra traffic is carried by the ring, achieving a bandwidth multiplication effect.
OCh/SPRING implements bi-directional protection switching. Bi-directional protection switching refers to a protection switching architecture where for a unidirectional failure (i.e., a failure affecting only one direction of the transmission), both directions, including the affected direction and the unaffected direction, are switched to the protection. Upon the detection of failure, OCh/SPRING requires a signaling protocol, an Optical Automatic Protection Switching (O-APS) protocol, to coordinate the switching from the working channels to the protection channels between the two termination nodes.
There exists a need for a ring-specific O-APS protocol. There have been some recent attempts to design protection mechanisms for generalized mesh topologies and then apply them to ring topologies, with the argument that a ring topology is just a degenerated case of mesh. In this approach the required protocols and algorithms for protection are not designed for the specific case of a ring topology. Although it is true that such mechanisms can be applied to ring topologies, from a practical perspective, this is not the best possible approach. For example, in the appendix we show the performance and scalability issues that result of applying a proposed mechanism called Fast Reroute Protocol (FRP) [BALA]—which we use as an example of restoration mechanism designed for generalized mesh topologies—to an optical ring topology.
In that analysis, it is shown that imposing the solution proposed in [BALA] onto an optical ring that supports OCh/SPRING results in a linear growth of the protection switching time subject to packet processing delay at each nodes and to the time for activating protection at each nodes—that will be referred as nodal protection activation time—as well as the number of nodes on the ring. Given the fact that OCh/SPRING technology allows wavelength reuse, the linear growth of the protection switching time will limit the ring size in terms of number of nodes that can reside on rings and the traffic demands that a ring can accommodate so as to meet the strict protection time requirement. It will also force one to increase the number of interconnected rings for a given set of traffic demands while alternately one could have larger rings with less number of interconnected rings, which is considered to have more dramatic cost implications.
The present invention is therefore directed to the problem of developing an optimum technique for performing protection switching in an optical network that does not limit the size or number of nodes in a protection path yet switches as rapidly as possible.